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diff --git a/testing/web-platform/tests/css/css-text/animations/letter-spacing-composition.html b/testing/web-platform/tests/css/css-text/animations/letter-spacing-composition.html
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+++ b/testing/web-platform/tests/css/css-text/animations/letter-spacing-composition.html
@@ -0,0 +1,53 @@
+<!DOCTYPE html>
+<meta charset="UTF-8">
+<title>letter-spacing composition</title>
+<link rel="help" href="https://drafts.csswg.org/css-text-3/#letter-spacing-property">
+<meta name="test" content="letter-spacing supports animation by computed value type">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/css/support/interpolation-testcommon.js"></script>
+
+<body>
+<script src="../interpolation/resources/interpolation-test.js"></script>
+<script>
+test_composition({
+  property: 'letter-spacing',
+  underlying: '50px',
+  addFrom: '100px',
+  addTo: '200px',
+}, [
+  {at: -0.3, expect: '120px'},
+  {at: 0, expect: '150px'},
+  {at: 0.5, expect: '200px'},
+  {at: 1, expect: '250px'},
+  {at: 1.5, expect: '300px'},
+]);
+
+test_composition({
+  property: 'letter-spacing',
+  underlying: '100px',
+  addFrom: '10px',
+  addTo: '2px',
+}, [
+  {at: -0.5, expect: '114px'},
+  {at: 0, expect: '110px'},
+  {at: 0.5, expect: '106px'},
+  {at: 1, expect: '102px'},
+  {at: 1.5, expect: '98px'},
+]);
+
+test_composition({
+  property: 'letter-spacing',
+  underlying: '50px',
+  addFrom: '100px',
+  replaceTo: '200px',
+}, [
+  {at: -0.3, expect: '135px'},
+  {at: 0, expect: '150px'},
+  {at: 0.5, expect: '175px'},
+  {at: 1, expect: '200px'},
+  {at: 1.5, expect: '225px'},
+]);
+</script>
+</body>